Demands for compact electric and electronic devices such as mobile communication devices and the like are more and more growing today, and the production of secondary batteries usable for these devices is also growing. Among the secondary batteries, the production amount of lithium secondary batteries (also referred to as “lithium ion secondary batteries”) having a high energy density is conspicuously growing. As applications of the electric and electronic devices is now diversified and efforts are being made to reduce the size thereof, the secondary batteries are desired to have more improved performance. Specifically, it is strongly desired to increase the discharge capacity and extend the life.
In lithium secondary batteries commercially available today, a Li-containing composite oxide such as LiCoO2 or the like is used for a positive electrode and graphite is used for a negative electrode (negative electrode active substance). However, since a negative electrode formed of graphite can occlude lithium ions only up to a composition of LiC6, the capacity per volume of lithium ions which can be occluded and released is theoretically 372 mAh/g at the maximum. This value is merely about ⅕ of the theoretical capacity of lithium metal. In other words, the capacity of lithium secondary batteries commercially available today is merely about ⅕ of the capacity of a lithium secondary battery using lithium as a negative electrode.
As substances capable of reversibly occluding or releasing lithium ions, metallic elements such as Al, Ga, In, Si, Ge, Sn, Pb, As, Sb, Bi and the like and alloys thereof are known. The theoretical capacities per volume thereof are, for example, as follows, and are all greater than the capacity per volume of a carbonaceous material such as graphite or the like.
Si: 2377 mAh/cm3 
Ge: 2344 mAh/cm3 
Sn: 1982 mAh/cm3 
Al: 2167 mAh/cm3 
Sb: 1679 mAh/cm3 
Bi: 1768 mAh/cm3 
Pb: 1720 mAh/cm3 
However, these materials significantly expand or contract at the time of reaction of occluding or releasing lithium ions. As shown in FIG. 10, a negative electrode of a lithium secondary battery generally includes a current collector 3 and a negative electrode material 101 for occluding lithium ions, which is attached to the current collector 3. Such a structure causes a problem that the negative electrode material 101 is delaminated from the current collector 103 by expansion and contraction thereof, resulting in capacity decrease.
To overcome this problem, Patent Document 1, for example, proposes a porous firm body formed of a ceramic material in which the pores are filled with silicon. Such a porous body is a frame for holding silicon, which is an expandable and contractable negative electrode active substance, with a mechanical strength thereof. As such a ceramic material, Patent Document 1 discloses using ceramics containing a substrate of one, or at least two, of carbides, borides, nitrides and oxides of an element selected from Be, Mg, Ti, Zr, V, Nb, Cr, Fe, Co, Ni, B, Al and Si.
In relation with the above-described problem, Patent Document 2 discloses that in order to guarantee electric contact between the substances even when the film of the active substance is ruptured, a current collector having a Cu/Sn/Cu structure is formed on a polyester film substrate and a film of an active substance is formed thereon
In order to realize a laminate structure such as a structure of a carbon material and suppress further pulverization of active substance powder during a charging/discharge process, Patent Document 3 discloses forming a laminate structure of an active substance layer and a Cu—Sn layer.
Patent Document 4 proposes a laminate structure in which a layer formed of a metal material having a low ability of forming a lithium compound is interposed between layers formed of Sn, Al, Sn alloy or Al alloy having a high ability of forming a lithium compound.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-090922
Patent Document 2: Japanese Patent No. 3755502
Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-168425
Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-139954